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Help choosing plywood for table tops - warping?

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Forum topic by Neophyte posted 07-16-2018 02:38 AM 1282 views 0 times favorited 49 replies Add to Favorites Watch
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Neophyte

32 posts in 2436 days


07-16-2018 02:38 AM

Friends, I am hoping to get some advice on plywood or other suitable material to build several square bistro table tops for a restaurants of some friends. They will replace the existing table tops. The current tables are made of two sheets of plywood glued together to 1.25” thickness and have pine moulding. Total dimension is 24×24 in. I built prototypes by glueing 3/4 + 1/2 in birch plywood and then trimming them to size (22.5 in per side). I added 3/4 poplar moulding. The problem is that the prototypes are not flat, most of them exhibit some warping. The table tops, by the way, will be painted.
I am looking for recommendations on which material to use, or how to use it. I have thought of trying Baltic birch plywood or a Spanish product called Garnica; glueing so that the grain is perpendicular and so forth. I will appreciate hearing what people know about this topic.
Marc

-- Marc, NY


49 replies so far

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jonah

1820 posts in 3378 days


#1 posted 07-16-2018 03:25 AM

If I’m understanding you, the tables will be 22.5” square?

The thing with gluing plywood together for thickness is that once the water in the glue gets into the wood, it swells and warps, and with two pieces trying to move, things get uneven in a hurry.

The solution is a lot of very even clamping pressure and the right amount of glue. Many people use a vacuum press for something like this, just like they would with veneer. There are a few ways to make your own vacuum press, but I’ve never done it so I can’t speak to that.

If a press isn’t an option, make sure you use very beefy, very straight cauls and a lot of clamping pressure spread out over the whole top.

An alternative would be to make the tops out of solid wood. That would be a little more work, but it’d be easier to flatten if it did end up warping since you could just take a hand plane to it.

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woodbutcherbynight

5486 posts in 2489 days


#2 posted 07-16-2018 03:38 AM

Made a mock up for a friends wife for some tables your size. Used 3/4 MDF, glued 5/8 wood flooring to the top and went around the edges with some pine to match the top. Very stable, and even if you had to level it you had real wood on the top.

-- Live to tell the stories, they sound better that way.

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jbay

2485 posts in 979 days


#3 posted 07-16-2018 03:52 AM

Biggest thing is to have it clamped to a flat surface.
If it’s not flat as it’s drying it won’t be flat when it’s dry.

Has to be clamped up flat…

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Rich

3333 posts in 669 days


#4 posted 07-16-2018 04:28 AM


Biggest thing is to have it clamped to a flat surface.
If it s not flat as it s drying it won t be flat when it s dry.

Has to be clamped up flat…

- jbay

For sure.

For long term I would think some battens underneath will help with stability as well, and give you something to mount to when you attach it to whatever base you’re planning.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

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bondogaposis

4889 posts in 2431 days


#5 posted 07-16-2018 04:37 AM

Tell us about the bases. You should be be able to pull a minor amount of warp out when you attach it to the base.

-- Bondo Gaposis

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Neophyte

32 posts in 2436 days


#6 posted 07-16-2018 04:51 AM



If I m understanding you, the tables will be 22.5” square?

The thing with gluing plywood together for thickness is that once the water in the glue gets into the wood, it swells and warps, and with two pieces trying to move, things get uneven in a hurry.

The solution is a lot of very even clamping pressure and the right amount of glue. Many people use a vacuum press for something like this, just like they would with veneer. There are a few ways to make your own vacuum press, but I ve never done it so I can t speak to that.

If a press isn t an option, make sure you use very beefy, very straight cauls and a lot of clamping pressure spread out over the whole top.

An alternative would be to make the tops out of solid wood. That would be a little more work, but it d be easier to flatten if it did end up warping since you could just take a hand plane to it.

- jonah

Yes, they are a 22.5” square, then add the moulding.
This is a great reply. Thanks.

-- Marc, NY

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Neophyte

32 posts in 2436 days


#7 posted 07-16-2018 04:54 AM


Tell us about the bases. You should be be able to pull a minor amount of warp out when you attach it to the base.

- bondogaposis

They look like this

-- Marc, NY

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Woodknack

12213 posts in 2460 days


#8 posted 07-16-2018 05:06 AM

Don’t use a Water-based glue, use contact cement or a polyurethane glue.

-- Rick M, http://thewoodknack.blogspot.com/

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Desert_Woodworker

1520 posts in 1294 days


#9 posted 07-16-2018 05:18 AM

I see that 2 of the finest LJocks have offered their expertise. Here are 2 pics from the net

Should you decide to clamp the 2 pc of plywood-
For clamping, from the underside use screws.

-- Desert_Woodworker

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Desert_Woodworker

1520 posts in 1294 days


#10 posted 07-16-2018 05:21 AM



Tell us about the bases. You should be be able to pull a minor amount of warp out when you attach it to the base.

- bondogaposis

+1

-- Desert_Woodworker

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Desert_Woodworker

1520 posts in 1294 days


#11 posted 07-16-2018 05:32 AM

Next question? What is the placement, while gluing, the 2 pc of plywood.
should, 1 pc be placed 90 degrees to the other or not?

-- Desert_Woodworker

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clin

907 posts in 1076 days


#12 posted 07-16-2018 06:17 AM

Another option is to screw the two pieces together from the bottom. You’d be able to get a lot more clamping pressure that way than you’d get without using some sort of press.

I also agree with Rick, that contact cement would be a good idea to consider.

I’d also go with 3/4” thick for both pieces. Or use whatever, but have them both the same thickness and same material. I think I’d run the grain of the two pieces (grain of the outside ply of course), in the same direction. That way the inner layers would be glued with grain aligned and the total plies would remain symmetrical. Sort of like the standard odd number of plies, except the inner one would be 2X thick.

Symmetry is important so that whatever happens creates the same forces to cancel out. This is another reason to be sure to finish the top and bottom so that rates of moisture gain and loss will tend to be the same. Though of course table tops get a lot of moisture during use and cleanup.

Certainly Baltic Birch would be very high quality and my experience has been that it tends to be very flat. But a lot depends on how it has been stored. I’ve only known it to come in 5’x5’ sheets, so you’d have some waste. Though you could use two smaller pieces on the bottom where it doesn’t show and reduce the waste significantly. But you’d lose much of the strength that way.

-- Clin

View John Smith's profile

John Smith

1229 posts in 242 days


#13 posted 07-16-2018 11:23 AM

when I make plywood boat transom panels, I find the flattest spot on the floor
I can find, lay down some newspaper, apply a hefty coating of epoxy to both mating
surfaces. put two drywall screws along one edge to prevent slippage, and apply
as much weight as I can find and let it sit overnight. (no clamps involved).
cut to shape the next day. I don’t cut the panels to shape first and then laminate them.
this eliminates any slippage issues.
the transom panels range in thickness from 1” to 2-1/2” with several pieces of plywood
laminated together. the veneer (if used) is applied after the lamination process.
unless you are making a rocket pad, there is no reason to alternate the grain pattern
in plywood for a 2×2’ panel – but it will definitely make it stay flat after it is all said and done.
[with epoxy – there is no issue of transferring moisture to the plywood panels].

when making transom panels for a boat that will have a 200 pound (or more) outboard motor
hanging on it for years and exerting an enormous amount of stress from all angles,
you can not afford to have any flex at all. with epoxy as the sealer, it is pretty much
weatherproof from the Bearing Sea to the Equator. so it should be okay for a table top
for you to enjoy your Tea and Crumpets on your patio.

to start with: think outside the box and get away from the mindset of using the finest
“interior plywood and glue” and go to the more extreme EXTERIOR grade plywood and epoxy.
after it is all laminated together, cut to your 22.5” square panels. fill the edge end grain
and any voids with epoxy. after a 24 hour cure, sand smooth, apply your edge banding,
and paint with your choice of exterior paint.

.

-- Graduated Valedictorian from the University of HardKnocks --

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Tony1212

210 posts in 1814 days


#14 posted 07-16-2018 01:38 PM



Another option is to screw the two pieces together from the bottom. You d be able to get a lot more clamping pressure that way than you d get without using some sort of press.

- clin

+1

When doing a glue up like this, I would use MDF as the bottom layer and the plywood on top. Then use some drywall screws through the MDF into the plywood. That provides a secure hold and LOTS of clamping pressure. This is how I made my workbench top and tablesaw cart top.

When both sides would be seen, I would put the glue up between two larger plywood sheets on the garage floor and park the front wheel(s) of my car on it. This works very well if there is any cupping in the plywood sheets. Orient them so that the glue up is wider in the center and the edges meet. Then the car will press the center flat.

-- Tony, SW Chicago Suburbs

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builtinbkyn

2619 posts in 1020 days


#15 posted 07-16-2018 01:57 PM


Don t use a Water-based glue, use contact cement or a polyurethane glue.

- Woodknack


That’s the way I’d go. Especially the contact cement. No issues with squeeze-out, no clamping necessary and you can use the tops immediately. Oh no screws required either.

-- Bill, Yo!......in Brooklyn & Steel City :)

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